In the maintenance of high quality golf courses it is critical that the height of the grass, particularly the greens, be maintained extremely accurately. Modern greens mowers are generally gang mowers with three or more mowing heads and it is important that each head should cut at precisely the same height as its neighbours to within a very small tolerance which is of the order of a few thousandths of an inch, in order to maintain an accurate and true playing green without any ridges to affect the play of the ball. Even a ridge as small as a few thousandths can be detected quite readily by an expert golf player. There are, of course, measuring devices which can be adapted to a mowing head but generally such devices are awkward to use as they require dissassembly of the gang mower so that each head can be independently set or, at the very least inversion of the relatively heavy mowing head. these devices usually comprise a flat bar, which is placed over the front and rear rollers of an inverted mowing head which preferably has been removed from the mower, and a feeler which passes through the flat bar and contacts the front edge of the bedknife. The front roller can then be adjusted to give the desired cutting height as indicated by manual feel. Not only is this cumbersome to use in that at least two measurements per head must be taken to ensure that both sides of the blade are level but also the mowing head is heavy and difficult to invert to apply the bar and guage. The operation is, therefore, very time consuming. There is, therefore, a need for a simple and efficient device for measuring the cutting height of a multi-head greens mower which can be operated without either inverting or removing the mowing head from the mowing machine.